The present invention relates to a beam scanning apparatus for two-dimensionally scanning over a region which is an object of measurement through use of an optical beam, such as a laser beam.
A commonly employed apparatus for measuring the distance between vehicles (hereinafter referred to simply as a "inter-vehicle distance measurement apparatus") basically comprises an optical beam scanning device for two-dimensionally scanning over a forward region which is an object of measurement (hereinafter referred to as a "forward region") through use of a laser beam; a light-receiving device for receiving reflected light of the thus-emitted laser beam; and a computation device for computing the distance between vehicles on the basis of the characteristics of the light received by the light-receiving device. For example, Japanese Patent Publication No. 9-274076 discloses such an apparatus for measuring the distance between vehicles.
In the related apparatus used in the inter-vehicle distance measurement apparatus, pulse light originating from a light source, such as a laser diode, is emitted as an optical beam by way of a collimator lens, and the thus-emitted optical beam is radiated onto a reflection plate which is pivotally moved within a given angular range or onto a rotating polygon mirror. The forward region is two-dimensionally scanned through use of the beam reflected from the reflection plate or the polygon mirror. For example, the inter-vehicle distance measurement apparatus described in the cited patent publication performs two-dimensional scanning operation through use of a polygon mirror.
The related apparatus is arranged so as to perform a scanning operation within a predetermined angular range through use of an optical beam, by means of pivotal movement of the reflection plate or by means of rotation of a polygon mirror. Means for deflecting an optical beam through use of such as a reflection plate or a like element requires a wider area for placement than does another constituent component, thus raising a problem in rendering the optical beam scanning apparatus compact.
Further, the related apparatus radiates an optical beam in the form of a pulse signal in synchronism with the angle of scanning. If an offset arises between the timing at which an optical beam is radiated (or illuminated) and the rotational angle (i.e., the angle of scanning) of a reflection plate or a polygon mirror, the optical beam scanning apparatus encounters a problem of inability to fully scan over the entire region which is an object of measurement. In order to work around such a problem, the first related apparatus actuates a reflection plate or a polygon mirror while maintaining a high degree of linearity relative to the timing at which an optical beam is to be radiated. To this end, a mechanism for actuating the reflection plate or the polygon mirror requires positional accuracy with a high degree of resolution.